Probing the Evolutionary History of Comets: An Investigation of the Hypervolatiles CO, CH4, and C2H6 in the Jupiter-family Comet 21P/Giacobini-Zinner
Résumé
Understanding the cosmogonic record encoded in the parent volatiles stored in cometary nuclei requires investigating whether evolution (thermal or otherwise) has modified the composition of short-period comets during successive perihelion passages. As the most volatile molecules systematically observed in comets, the abundances of CO, CH 4 , and C 2 H 6 in short-period comets may serve to elucidate the interplay between natal conditions and post-formative evolution in setting present-day composition, yet secure measurements of CO and CH 4 in Jupiterfamily comets (JFCs) are especially sparse. The highly favorable 2018 apparition of JFC 21P/Giacobini-Zinner enabled a sensitive search for these "hypervolatiles" in a prototypical carbon-chain depleted comet. We observed 21P/Giacobini-Zinner with the iSHELL spectrograph at the NASA Infrared Telescope Facility on four preperihelion dates, two dates near-perihelion, and one post-perihelion date. We obtained detections of CO, CH 4 , and C 2 H 6 simultaneously with H 2 O on multiple dates. We present rotational temperatures, production rates, and mixing ratios. Combined with previous work, our results may indicate that the hypervolatile coma composition of 21P/ Giacobini-Zinner was variable across apparitions as well as within a particular perihelion passage, yet the spread in these measurements is a relatively small fraction of the variation in each molecule from comet to comet. We discuss the implications of our measured hypervolatile content of 21P/Giacobini-Zinner for the evolution of JFCs, and place our results in the context of findings from the Rosetta mission and ground-based studies of comets.
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